It is currently 23 Feb 2018, 19:56

### GMAT Club Daily Prep

#### Thank you for using the timer - this advanced tool can estimate your performance and suggest more practice questions. We have subscribed you to Daily Prep Questions via email.

Customized
for You

we will pick new questions that match your level based on your Timer History

Track
Your Progress

every week, we’ll send you an estimated GMAT score based on your performance

Practice
Pays

we will pick new questions that match your level based on your Timer History

# Events & Promotions

###### Events & Promotions in June
Open Detailed Calendar

# If f(x) = x^3 + kx^2 − 4x, what is the value of k?

 new topic post reply Question banks Downloads My Bookmarks Reviews Important topics
Author Message
TAGS:

### Hide Tags

Math Expert
Joined: 02 Sep 2009
Posts: 43892
If f(x) = x^3 + kx^2 − 4x, what is the value of k? [#permalink]

### Show Tags

06 Aug 2017, 00:04
Expert's post
1
This post was
BOOKMARKED
00:00

Difficulty:

55% (hard)

Question Stats:

58% (01:45) correct 42% (01:48) wrong based on 42 sessions

### HideShow timer Statistics

If f(x) = x^3 + kx^2 − 4x, what is the value of k?

(1) f(k) = −f(−k)
(2) f(f(2)) = 0
[Reveal] Spoiler: OA

_________________
Manager
Joined: 02 Nov 2015
Posts: 174
GMAT 1: 640 Q49 V29
Re: If f(x) = x^3 + kx^2 − 4x, what is the value of k? [#permalink]

### Show Tags

06 Aug 2017, 00:56
Statement 1 alone is sufficient IMO.
As per statement 1: -f(-k) yields x^3-kx^2+4x.
So x^3+kx^2+4x=x^3-kx^2+4x.
Which ultimately yield 0.

So answer.
Statement 2 gives different values of K.

Sent from my Lenovo TAB S8-50LC using GMAT Club Forum mobile app
Director
Joined: 18 Aug 2016
Posts: 628
Concentration: Strategy, Technology
GMAT 1: 630 Q47 V29
GMAT 2: 740 Q51 V38
Re: If f(x) = x^3 + kx^2 − 4x, what is the value of k? [#permalink]

### Show Tags

06 Aug 2017, 02:40
Bunuel wrote:
If f(x) = x^3 + kx^2 − 4x, what is the value of k?

(1) f(k) = −f(−k)
(2) f(f(2)) = 0

(1)
$$f(k) = 2k^3 - 4k = -f(-k) = -4k$$
$$k^3 = 0$$
$$k=0$$

Sufficient

(2)
$$f(2) = 4k$$
$$f(f(2)) = 80k^3 -16k = 0$$
$$k = 0,+1/\sqrt{5}, -1/\sqrt{5}$$
Not sufficient

Hence A
_________________

We must try to achieve the best within us

Thanks
Luckisnoexcuse

Re: If f(x) = x^3 + kx^2 − 4x, what is the value of k?   [#permalink] 06 Aug 2017, 02:40
Display posts from previous: Sort by

# If f(x) = x^3 + kx^2 − 4x, what is the value of k?

 new topic post reply Question banks Downloads My Bookmarks Reviews Important topics

 Powered by phpBB © phpBB Group | Emoji artwork provided by EmojiOne Kindly note that the GMAT® test is a registered trademark of the Graduate Management Admission Council®, and this site has neither been reviewed nor endorsed by GMAC®.